Method of separating materials of different density



Dec. 16, 1969 METHOD OF SEPARATING MATERIALS OF DIFFERENT DENSITY FiledJune 12, 19s? R. KAISER 3,483,968

2 Sheets-Sheet l GLASS v CERAMIC CORAL CLOSED M dH -XIO 3 dynes /cc IF mROBERT KAISER INVENTOR.

ATTORNEYS R. KAISER Dec. 16, 1969 METHOD OF SEPARATING MATERIALS OFDIFFERENT DENSITY Filed June 12, 1967 2 Sheets-Sheet 2 zwmo JA E ommO aJA E QMmOJO JA E ROBERT KAISER INVENTOR.

ATTORNEYS 6 wumaowdz United States Patent US. Cl. 209-1 7 ClaimsABSTRACT OF TI-IE DISCLOSURE Method of separating materials of differentdensity by passing a controlled magnetic field through a ferrofluid andintroducing the materials into the ferrofluid.

DEFINITIONS For the purposes of this discussion, a ferrofluid is definedas a material comprising a permanent colloidal suspension offerromagnetic particles in a liquid carrier. The particles do notseparate from the liquid carrier in the presence of a magnetic,gravitational, or acceleration field. The composite which is comprisedof carrier fluid and particles appears to have the property of magneticpolarizability that is uniform Additional information relating to thesubject matter of this invention may be found in the co-pendingapplication entitled, Means For and Method of Moving Objects byFerrohydrodynamics, Ser. No. 487,520, filed Sept. 15, 1965, to the sameassignee as this application.

For the purposes of this discussion, levitation shall be understood tomean the effect of raising a body in contact with or submersed inferrofluid in a direction opposite to gravity.

BACKGROUND OF THE iNVENTION In general, this invention is directed to aprocess of separating materials of different density and moreparticularly to accomplishing this objective through the use of magneticfields and ferrofiuids.

OBJECTS It is an object of the invention to provide a novel method ofseparating materials according to differences in density.

It is another object of the invention to provide a method of separatingmaterials of different density using the interaction of magnetic fieldsand ferrofiuids.

It is yet another object of the invention to achieve separation ofmaterials of different density by differential levitation wherebymaterials of different density occur in different elevational strata.

It is still another object of the invention to achieve separation ofmaterials by sequential levitation whereby materials of differentdensity are levitated in sequence in accordance with their respectivedensities.

It is yet another object of the invention to achieve separation ofmaterials of different density which is a function only of the densityand independent of the shape and size of the material.

The invention, both as to its organization and method of operation,together with additional objects and advantages thereof, will best beunderstood from the following description of a specific embodiment whenread in conjunction with the accompanying drawings, in which:

FIGURE 1 is a curve useful in describing the theory of operation.

FIGURE 2 is a schematic representation of process steps involved inseparating materials of different density by sequential levitation.

ice

FIGURE 3 is a schematic representation of a process step useful inadapting a portion of FIGURE 2 to convert the FIGURE 2 process into aprocess for separating materials of different density by differentiallevitation.

THEORY OF OPERATION The purpose of this invention is to separatematerials of different density using the principle of levitation in aferrofluid. While the process and theory will be discussed in terms ofsolid particles, the process will work with immiscible liquids ofdifferent density.

The following equation was developed from the force equation presentedin the co-pending patent application identified above. It described theequilibrium gravitational and magnetic forces acting on a non-magneticobject in a ferrofluid in the presence of a magnetic field having avertical gradient.

Z7;d Z (Equation 1) In the above equation:

=density of the solid,

=density of the liquid,

gzacceleration of gravity=981 cm/sec.

M=average magnetization of liquid displaced by solid particle, gaussdH/dZ=gradient of magnetic field oersted/cm.

The gravitational term in Equation 1 (the left-hand term) is determinedonly by the difference in density of the solid object being levitatedand density of ferrofluid used. For any given system both of theseparameters are known. The magnetic (right-hand) term of Equation 1 isuniquely determined by the height, Z, above an arbitrary reference line.Since H is also a function of Z so is dH a function of Z. For a givenferrofluid, the magnetization of the liquid, M, is determined by thelocal field H, so it too, is solely a function of Z.

In practice, it is important to recognize that there are tWo specificrequirements for the magnetic field. Frst, it must be strong enough toresult in a noticeable magnetization of the fluid and secondly, themagnitude of this field must decrease in the upward direction so as tocreate a vertical gradient dH/dZ.

It would appear when a mixture of solid objects of different density isimmersed in a ferrofluid and such a magnetic field applied, such ofthese solid objectsor non-magnetic immisible fluids-of different densitycan be selectively levitated. At least two situations have become clear.If dH/dZ is uniform throughout the ferrofluid, the levitation forcesincrease with an increase in MdH/dZ. If M is a saturation value as willmost often be the case, the levitation force is purely a function ofdH/dZ. Accordingly, as dH/dZ is increased, objects of lowest densitywill be levitated first to be followed in a sequence by objects ofgreater density in order of their relative densities.

On the other hand, if the magnetic field has a nonhomogeneous gradient,dH/dZ opposed to the direction of gravity, that is to say the magnitudeof dH/dZ decreases in the Z direction opposite to the gravitationalfield, objects of different density will seek different levels in thefluid such that in each case Equation 1 is satisfied. Under theseconditions, objects of lesser density will be elevated above objects ofgreater density. In other words, objects seek levels of elevation thatare a function of their relative densities. There are two extreme caseswhere there can be no separation, namely when all the objects arelevitated to the surface or when none of the objects are levitated atall.

A prepared mixture of solid objects of different specific gravities(densities) was immersed in a ferrofluid in the presence of a magneticfield gradient opposed to the direction of gravity. The objects immersedWere:

Material: S.g. Glass 2.35

Ceramic 2.72

Diamonds 3.40 Sapphire 3.96 Coral 5.46

These solid objects floated at different levels which allowed theobjects to be easily separated. It should be noted that objects ofdifferent sizes and shapes in each of the materials were used in theseexperiments and that these parameters did not influence the results. Thecurve in FIGURE 1 summarizes the results demonstrated experimentally.The curve 11 in FIGURE 1 represents a theoretical prediction based onEquation 1. The discrete points taken with three different objectshaving three different densities is seen to correlate very closely withthe prediction.

A magnetic levitation effect is most pronounced and most effective whenperformed with nonmagnetic materials. The magnetic levitation effectwill be smaller with solid objects that are magnetically responsive thanwith magnetically non-responsive materials. Magnetically responsivesolids will behave as non-magnetic objects of higher density. Themagnetic lift term will be equal Where M is the volume averagemagnetization of the solid. When M =0, this reduces to Equation 1. WhenM M, there is no magnetic levitation effect possible. The presence ofstrongly magnetic substances (i.e., M M) in the batch of materials to beseparated has very little deleterious effect on the process describedsince the material will be attracted to the magnetic field and made aninactive constituent.

GENERAL DISCUSSION While it was known previously that non-magneticobjects could be levitated, the unique developments embodied in theinvention covered by this discussion was the realization thatferromagnetic levitation could be adapted to solve a problem ofseparating materials according to their density. Further contributionwas the realization that this could be done through the manipulation ofdH/dZ in particular and MdH/dZ in general. Both sequential levitationand differential leviation represents an important contribution toadvancing the state of the art as it existed prior to the invention.

SEQUENTIAL LEVITATION Separation of objects of different density bymeans of sequential levitation is best described by reference to FIGURE2. The common elements of FIGURE 2A through FIGURE 2E are a container 12in which a quantity of ferrofluid 13 is inserted. Extending from oneside of the container 12 is a pair of screens 14 and 16, respectively.These screens may be moved laterally, to the right in this particularconfiguration, to block the upward or downward movement of particlesthat may be placed Within the container 12. At the bottom of thecontainer 12 is found a valve 17 which when open serves to drain theferrofluid 13 from the container 12. A magnetic field source is shownschematically at 18.

In accordance with one aspect of the theory previously discused, themagnetic field source 18 when energized will provide a uniform dH/dZwithin the ferrofluid 13. Finally, objects having different densitiesare immersed in the ferrofluid 13 as shown in particular in FIGURE 2A.These objects 19 are further assumed to have densities varying from verylow to high. In this case since all of the objects 19 are found at thebottom of the container 12, the density of the least dense objects inthis 4 case obviously is higher than the density of the ferrofluid 13.

In FIGURE 2A all of the objects 19 rest on the bottom of the container12 and the magnetic field source is de-energized as indicated. In FIGURE2B the magnetic field source has been turned on. There is created withinthe ferrofluid 13 a value of of sufficient magnitude to cause the lowdensity particles 21 to float to the top of the ferrofluid 13. In thiscase, a uniform gradient dH/dZ is produced. This is symbolically shownat 20 in FIGURE 2B. It must be emphasized that the low density particles21 need not necessarily be homogeneous in their composition, but theirdensities must be such that they all are less than or equal to thedensity of the densest particle floating in the ferrofluid. At thispoint, the screen 14 is moved to the right and placed in a position toprevent any further particles that may Want to rise from rising past theheight of the screen 14.

Before proceeding, an assumption has been made that M does not vary.This will apply to most instances where an electromagnet is the magneticfield source and that the magnetic field generated results in saturationof the ferrofluid.

In FIGURE 2C the following conditions exist. The magnetic field sourceis turned on but its density is such that is higher than that whichexists in connection with FIG- URE 2B. Under these conditions, themedium density particles 22 are levitated but for the presence of thescreen 14 in this case, the medium density particle 22 noted will riseto the surface of the ferrofluid 13. Here again, the common element inthe medium density particle 22 is that all of these are less dense thanthe densest particle levitated.

With the magnetic field source still on, the screen 16 is now moved tothe right separating the medium density particles 22 from the heavydensity particles 23, Finally, the magnetic field source is turned off.Clearly, in the absence of magnetic field, the levitated particles 21and 22 tend to fall toward the bottom of the container 12 and areprevented from doing so by the screens 14 and 16. The valve 17 is nowopen draining the ferrofluid 13 from the container 12 and it is clearfrom FIGURE 2C that a complete separation has been achieved, in thiscase into three cuts, so to speak, of the original mixture of objects 19(FIGURE 2A), according to the respective densities of the particles.

It is clear that by providing a larger number of screens and by more.accurate control of the magnetic field it would appear possible toseparate the original mixture of the objects 19 into a greater number ofcuts. Generally, however, as the process is conceived, three will beadequate. For example, if one wanted to recover a valuable componentsuch as diamonds which exist as a trace commodity in the dirt betterknown as gangue, in which it exists in a material state, one wouldadjust the system shown schematically in FIGURE 2 such that lightdensity gangue would float to the top, diamonds and those elements ofgangue of the same density as diamonds in the center with the heavygangue remaining on the bottom. The result is a mixture of diamonds andgangue in the center cut in which diamonds are found in a substantiallylarger concentration in the original gangue.

DIFFERENTIAL LEVITATION In the event the magnetic field generated by themagnetic field source is designed to have a non-homogeneous distributionof dH/dZ as more particularly shown in FIGURE 3 by a separation betweenthe dotted lines 24. It is clear from Equation 1 that the objects ofdifferent density will be levitated within the fluid to difierent levelsof elevation as shown in FIGURE 3. It is merely necessary now to insertthe screens 14 and 16 as described previously, drain the ferrofluid 13and remove the three cuts.

Summarizing the process briefly, a ferrofluid of known density andmagnetization is provided. A magnetic field having a magnetic fieldgradient in the opposite direction to gravity is passed through theferrofluid. A mixture of materials, preferably non-magnetic or lessmagnetic than the ferrofluid, of different density is placed in theferrofluid.

All of the mixture may float in which case (lg dZ is high. All of themixture may sink in which case (1H cZZ is small. It is quite clear thatintermediate situations are easily achieved by adjusting M, dH/dZ orboth.

This invention relates to a unique method of materials separation whichis based on the ability to control and change the apparent density ofthe ferrofluid both in space (differential levitation) and time(sequential levitation) by application of a properly designed magneticfield whose intensity and space gradients can be varied at will incombination with the proper application in time and space of mechanicaldividers.

It is to be emphasized that the proper sequence of events has to occurin the right order and the magnetic field has to be properly designed inorder for the object of this invention which is the separation ofmaterials of differing density to occur. For example, if a mixture ofsolid particles is placed in a bath of ferrofluid and a magnetic fieldgradient is applied which results in flotation of the densest particlesto the surface of the bath, levitation will have occurred but separationwill not have occurred.

What is claimed is:

1. A method of separating materials of different density comprising thesteps of:

(a) providing a ferrofluid material comprising a permanent colloidalsuspension of magnetic material in a liquid carrier, which colloidalsuspension does not separate from the liquid carrier in the presence ofmagnetic, gravitational or an acceleration field, of known density andmagnetization;

(b) passing a variable magnetic field through the ferrofiuid, said fieldhaving a magnetic field gradient dH/dZ in a vertical direction oppositeto gravity;

(c) inserting in the ferrofluid a mixture of materials of diflerentdensity; and

(d) adjusting the magnitude of at least the magnetic field gredientdH/dZ for positioning at least two materials of different density atdifierent elevations IVI in the ferrofluid with the lighter densitymaterial being above material of greater density.

2. A method of separating materials of diiferent density as described inclaim 1 in which the magnetic field intensity as well as its gradientare varied.

3. A method of separating materials of different density as described inclaim 1 in which the magnetic field gradient is uniform.

4. A method of separating materials of different density as described inclaim 1 in which the magnetic field gradient is nonuniform.

5. A method of separating materials of different density comprising thesteps of:

(a) providing a ferrofluid material comprising a permanent colloidalsuspension of magnetic material in a liquid carrier, which colloidalsuspension does not separate from the liquid carrier in the presence ofmagnetic, gravitational or an acceleration field, of known density andmagnetization;

(b) passing a variable magnetic field through the ferrofluid, saidfields having a magnetic field gradient dH/dZ in a direction opposite togravity;

(c) inserting in the ferrofluid a mixture of materials of differentdensity and adjusting the magnetic field intensity and gradient so thatall of the mixture of materials is at one elevation; and

(d) adjusting the magnitude of at least the magnetic field gradientdH/dZ for positioning materials of different density at difierentelevations in the ferrofluid with the lighter density material beingabove material of greater density.

6. A method as described in claim 5 in which said adjustment comprise asequence of changes for changing the elevation of constituents of saidmixture in sequence in accordance with their relative density.

7. A method as described in claim 5 in which dH/dZ is non-uniformproviding differential levitation and said adjustment sets the range ofdensity which will be levitated.

References Cited UNITED STATES PATENTS 2,902,153 9/1959 Green 2092083,065,640 11/1962 Langmuir 73-517 3,133,876 5/1964 Klass 209-1 3,206,9879/1965 Cunningham 73517 OTHER REFERENCES American Journal of Physics,vol. 33, No. 5, May 1965, pp. 406, 407; Electrostatic Separation, byLewis Epstein.

International Science and Technology, July 1966, Magnetic Fluids, pp.4854 and 56, R. E. Rosensweig.

FRANK W. LUTTER, Primary Examiner US. Cl. X.R. 209-172.5; 210-

